查找：生产ID为DTNTR物品的供应商，生产的其他物品。

子查询

SELECT
	prod_id,
	prod_name 
FROM
	products 
WHERE
	vend_id = ( SELECT vend_id FROM products WHERE prod_id = 'DTNTR' );

自联结

SELECT p1.prod_id,p1.prod_name   # 必须指定为p1.prod_id,p1.prod_name
FROM
	products AS p1,
	products AS p2 
WHERE
	p1.vend_id = p2.vend_id 
	AND p2.prod_id = 'DTNTR'

p2已经被限制为了具体的一行

SELECT p2.prod_id,p2.prod_name
FROM
	products AS p1,
	products AS p2 
WHERE
	p1.vend_id = p2.vend_id 
	AND p2.prod_id = 'DTNTR'

SELECT
	* 
FROM
	products AS p1,
	products AS p2 
WHERE
	p1.vend_id = p2.vend_id 
	AND p2.prod_id = 'DTNTR'

比较

从运行时间看，自联结比子查询快！